This invention relates to a tube fitting assembly for use in tubing employed, for example, in an industrial plant and in analytical equipment.
FIGS. 4 and 5 show an example of a tube fitting assembly. Since the devices shown in FIGS. 4 and 5 could represent both conventional devices and the devices in accordance with the present invention, the same reference numerals are used to show and describe the similar parts of both types of devices even though they are the elements of different devices. The assembly includes a tube 1, a coupling body 2, a front sleeve 3, a back sleeve 4, and an envelope nut 5. The back sleeve 4 is not of uniform hardness, with the forward end being hard and the rest of the piece being relatively softer. To assemble these components, first the envelope nut 5, then the back sleeve 4 and the front sleeve 3 are fitted in order around the outer perimeter of the tube 1. Next, the end of the tube 1 is inserted into the mouth of the coupling body 2, and the envelope nut at the rear is threaded onto the coupling body, turned and tightened with appropriate hardware.
However, the coupling body 2 is designed to restrict forward movement of the back sleeve 4 and front sleeve 3. Accordingly, the back sleeve 4 and front sleeve 3 cannot move in the axial direction beyond a prescribed distance, and further, the envelope nut 5 prevents their outward expansion. Therefore, the driving force of the envelope nut 5 is expanded towards the inner walls of the back sleeve 4 and front sleeve 3; in other words the driving force escapes by expanding in the direction of the tube 1. As a result, as shown in FIG. 6 which shows a conventional device, the inner walls of the front sleeve 3 and of the back sleeve 4 bite, albeit very minimally, into the surface of the tube 1.
Once the back sleeve 4 and front sleeve 3 bite into the surface of the tube 1, the tube is completely sealed in the fitting to resist the force of the internal pressure in the tube tending to loosen it, and a stable fitting can be maintained.
The conventional tube fitting assembly as described above has the following potential problem.
In order to provide the aforementioned functions, as described above, the back sleeve 4 is not of uniform hardness, wherein only the forward end is hard and the rest of the piece is relatively softer. Therefore, as shown in FIG. 6, as it is being tightened with the envelope nut 5, the relatively softer rear portion of the back sleeve 4 is the first to plastically deform, and as it is being compressed, applies pressure onto the front sleeve 3 in the axial direction of the fitting. In other words, the tube 1 is sealed and secured by an efficient use of this difference in hardness distribution of the back sleeve 4, namely with only the front tip being hard and the rest of the piece being relatively softer.
To obtain this difference in hardness distribution, conventionally, in fabricating the back sleeve 4, the front end thereof is carburized to increase its surface hardness. However, although this carburization will increase the surface hardness of the material, the process also markedly reduces its corrosion resistance. Accordingly, with progressive operation of the equipment, the surface of the back sleeve 4 of the tube fitting will gradually corrode, and eventually the corrosion product such as rust so formed could contaminate the material flowing through the tubing. This could result in erroneous results if the tubing is connected to equipment being used for inspection or other analytical work.
Moreover, the carburization process requires special equipment and skilled labor, and is ultimately uneconomical.